#include <stdio.h>
#define Result_t(T, E)                                                         \
  struct Result_##T##_##E {                                                    \
    bool is_ok;                                                                \
    union {                                                                    \
      T value;                                                                 \
      E error;                                                                 \
    };                                                                         \
  }

#define Ok(T, E)                                                               \
  (struct Result_##T##_##E) {                                                  \
    .is_ok = true, .value = (T)_OK_IMPL
#define _OK_IMPL(...)                                                          \
  __VA_ARGS__                                                                  \
  }

#define Err(T, E)                                                              \
  (struct Result_##T##_##E) {                                                  \
    .is_ok = false, .error = (E)_ERR_IMPL
#define _ERR_IMPL(...)                                                         \
  __VA_ARGS__                                                                  \
  }

typedef const char *ErrorMessage_t;

Result_t(int, ErrorMessage_t) my_func(int i) {
  if (i == 42)
    return Ok(int, ErrorMessage_t)(100);
  else
    return Err(int, ErrorMessage_t)("Cannot do the thing");
}

int main(void) {
  Result_t(int, ErrorMessage_t) x = my_func(42);

  if (x.is_ok) {
    printf("%d\n", x.value);
  } else {
    printf("%s\n", x.error);
  }
}